To guarantee interoperability between contactless card readers and transponders, international standards specify the properties of the air interface. For example, ISO/IEC 14443 is the fundamental international standard for proximity cards, ISO/IEC 10373-6 is the test standard for proximity systems, EMVCo is the industry standard for payment and ECMA 340 is the Near Field Communication (NFC) interface and protocol. Conformance of the contactless card readers and transponders to these standards is typically essential and in some instances needs to be certified by an accredited test laboratory. A number of properties are specified for the air interface of contactless products by the international standards. One property is the so-called Load Modulation Amplitude (LMA).
For example, in the communication link from a device in card mode (hereinafter referred to as the transponder device) to a device in contactless reader mode (hereinafter referred to as the contactless reader), the information is communicated using load modulation. Due to the inductive proximity coupling between the loop antenna circuit of the reader and the loop antenna circuit of the transponder device, the presence of the transponder device affects the contactless reader and is typically referred to as the “card loading effect”. From the perspective of the contactless reader, a change in resonance frequency and a decrease in the Quality (Q) factor of the resonant circuit occurs. If the contactless reader/transponder device coupling system is viewed as a transformer, the transponder device represents a load to the contactless reader. Modulating the frequency and Q of the transponder loop antenna circuit produces a modulation of the load on the contactless reader. The contactless reader detects this load modulation at the reader antenna as an AC voltage. For systems compliant with ISO/IEC 14443, for example, the load modulation is applied to a sub-carrier frequency (e.g. 0.8475 MHz) of the 13.56 MHz carrier frequency specified by the standard or the 13.56 carrier frequency is directly modulated by the encoded signal for systems compliant with FeliCa, a contactless RFID smartcard system developed by Sony in Japan.
Each standard typically specifies a minimum limit for the load modulation amplitude that needs to be achieved by the transponder device in card mode.
Typically, restrictions such as available space or cost place strict limits on the antenna size. Furthermore, the presence of other components in close proximity to the contactless reader antenna circuit or transponder device antenna circuit effect the antenna circuit resonance properties, typically producing a shift in resonance frequency and decreasing the Q-factor. To address this issue, typically ferrite materials such as sintered or polymer ferrite foils are used for one layer of the construction of transponder and reader antennas. For example, see US Patent Publication 201100068178 A1 incorporated by reference herein.
For transponder devices that are powered only by the contactless reader device, there is typically a physical limitation on the load modulation that may be achieved using conventional methods such as passive switching of a resistor or capacitor to modulate the frequency or Q-factor of the antenna resonance circuit. The physical limitation typically depends on antenna size of the transponder device, the coupling between transponder and reader, the Q-factor of the resonant circuit, the switching time and other parameters. Note, the switching time is fixed for the 847.5 kHz subcarrier frequency in context of the ISO/IEC 14443 standard. These physical limitations allow the generation of a limit curve for the minimum antenna area that can achieve compliance with the minimum load modulation specified by the standards.
The minimum load modulation required can be achieved using a smaller planar loop antenna if the card mode communication is transmitted actively into the contactless reader antenna. Options exist which can induce the same voltage into the contactless reader antenna as is possible using conventional passive amplitude load modulation. For example, one option is to transmit a 13.56 MHz carrier signal that is modulated by the 847.5 kHz subcarrier frequency which is in turn modulated using the encoded data operating in card mode.
However, for active load modulation to work, the active load modulation of the transponder device typically needs to be in phase with the, for example, 13.56 MHz alternating magnetic field emitted by the contactless reader. The contactless reader typically provides the time reference for communication using the contactless interface. Typical transponder devices derive the clock frequency from the exemplary 13.56 MHz carrier signal provided by the contactless reader. Therefore, the signal typically used for the communication link from the transponder device to the contactless reader is in phase with the carrier signal emitted by the contactless reader. For a transponder device actively emitting in card mode with only one antenna, however, it is typically not possible to obtain the time reference from the contactless reader carrier signal.